Surface-emitting semiconductor light emitting device

ABSTRACT

A surface-emitting semiconductor light emitting device comprises an n-type ZnSe buffer layer, n-type ZnSSe layer, n-type ZnMgSSe cladding layer, n-type ZnSSe waveguide layer, active layer, p-type ZnSSe waveguide layer, p-type ZnMgSSe cladding layer, p-type ZnSSe layer,p-type ZnSe contact layer, p-type ZnSe/ZnTe MQW layer and p-type ZnTe contact layer, sequentially stacked on an n-type GaAs substrate. A grid-shaped p-side electrode and a Au film convering the p-side electrode are provided on the p-type ZnTe contact layer. An n-side electrode is provided on the back surface of the n-type GaAs substrate. The active layer has a single quantum well structure or a multiple quantum structure including ZnCdSe quantum well layers.

This application is a division of Ser. No. 08/769,710, filed Dec. 18,1996, which is a continuation of Ser. No. 08/499,894, filed Jul. 11,1995 U.S. Pat. No. 5,617,446.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a surface-emitting semiconductor lightemitting device and, more particularly, to a surface-emittingsemiconductor light emitting device, such as semiconductor laser orlight emitting diode, capable of surface emission of green to bluelight.

2. Description of the Prior Art

Recently, demand has become greater and greater for semiconductor lightemitting devices capable of emitting green to blue light, aiming higherrecording and reproducing densities or higher resolutions of opticaldisks and magnet-optical disks, and efforts are devoted to developmentsof such devices.

U.S. Pat. No. 5,268,918 discloses a semiconductor light emitting devicehaving cladding layers of ZnMgSSe which is one of II-VI compoundsemiconductors.

Itoh et al. report, in Electronics Letters Vol. 29 No. 9, pulseoscillation of a semiconductor laser of an SCH structure made ofZnCdSe/ZnSe/ZnMgSSe at room temperature.

Ren et al. report, in J. Vac. Sci. Technol. B 12(2), March/April 1994 pp1262-1265, a LED having a double-heterostructure of ZnSe/ZnCdSe on aZnSe substrate.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the invention to provide a surface-emittingsemiconductor light emitting device capable of surface emission of greento blue light, using II-VI compound semiconductors as materials of itscladding layers and active layer.

According to the invention, there is provided a surface-emittingsemiconductor light emitting device, comprising: a first cladding layeron a substrate; an active layer on the first cladding layer; a secondcladding layer on the active layer; a first electrode electricallyconnected to the first cladding layer; and a second electrodeelectrically connected to the second cladding layer, in which the firstcladding layer, active layer and second cladding layer comprise II-VIcompound semiconductors, and light is emitted in a direction normal tothe plane of the active layer from one side of the active layer remoterfrom the substrate.

Since the device emits light in a direction normal to the plane of theactive layer, such light emitting devices can be integrated more easilythan light emitting devices configured to emit light in a directionnormal to the cleavage plane.

When a ZnTe compound semiconductor layer is stacked on the secondcladding layer and a grid electrode is provided on the ZnTe compoundsemiconductor layer, resistance of the ZnTe compound semiconductor layercan be greatly reduced by sufficiently increasing its carrierconcentration by doping an impurity, and good ohmic contact between theZnTe compound semiconductor layer and the grid-shaped electrode isensured. This permits a reduction of the operating voltage of thesurface-emitting semiconductor light emitting device and hence preventsheating at the contact of the grid electrode with the ZnTe compoundsemiconductor layer, which contributes to prevention of a decrease inefficiency of light emission or deterioration of the semiconductor lightemitting device. When the thickness of the ZnTe compound semiconductorlayer is in the range of 2 to 100 nm, the ZnTe compound semiconductorlayer behaves as a transparent electrode, and does not disturb emissionof light therethrough. When the ZnTe compound semiconductor layer issufficiently thick, such as 50 nm or more, for example, the currentinjected through the grid electrode can be diffused in the directionparallel to the plane of the ZnTe compound semiconductor layer. It isthus possible to introduce the current into a wide area of the activelayer and to realize excellently uniform surface emission of light.

When the grid electrode is made on the ZnTe compound semiconductorlayer, the current injected into the ZnTe compound semiconductor layerthrough the grid electrode is more uniformly distributed than in astructure using a stripe-shaped electrode, for example. Therefore,uniformity of surface emission of light can be improved.

When a ZnSSe compound semiconductor layer is provided between the secondcladding layer and the ZnTe compound semiconductor layer, also the ZnSSecompound semiconductor layer can be used as a cladding layer of a secondconductivity type so as to ensure excellent optical confinement andcarrier confinement characteristics. If the thickness of the ZnSSecompound semiconductor layer is around 0.5 μm, then the currentintroduced through the grid electrode can be diffused into a wide areain the direction parallel to the ZnSSe compound semiconductor layer, andhence ensures excellently uniform surface emission of light.

When a light reflecting layer is provided between the substrate and thefirst cladding layer, the light reflecting layer reflects a component oflight generated in the active layer and running toward the substrateback to the grid electrode, and prevents that the light toward thesubstrate is absorbed by the substrate. Therefore, it is possible to useeven the light toward the substrate for surface emission and henceimprove the light emitting efficiency remarkably. Light is thus takenout efficiently from the light take-up portion.

With these arrangements, a semiconductor light emitting device capableof surface emission of green to blue light can be realized, using II-VIcompound semiconductors as materials of its cladding layers and activelayer.

The above, and other, objects, features and advantage of the presentinvention will become readily apparent from the following detaileddescription thereof which is to be read in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a surface-emitting light emitting diode (LED)according to a first embodiment of the invention;

FIG. 2 is a cross-sectional view of the surface-emitting LED accordingto the first embodiment of the invention;

FIG. 3 is an energy band diagram showing valence bands of p-type ZnSeand p-type ZnTe near their interface;

FIG. 4 is a graph showing changes in first quantum level E₁ with widthL_(w) of a quantum well comprising p-type ZnTe;

FIG. 5 is an energy band diagram showing a design of a p-type ZnSe/ZnTeMQW layer in the surface-emitting LED according to the first embodimentof the invention;

FIG. 6 is a cross-sectional view of a surface-emitting LED according toa second embodiment of the invention; and

FIG. 7 is a cross-sectional view showing a surface-emitting LEDaccording to a third embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Some embodiments of the invention are explained below with reference tothe drawings. In all drawings of embodiments, the same or equivalentelements are labelled with identical reference numerals.

FIG. 1 is a plan view of a surface-emitting light emitting diode (LED)according to a first embodiment of the invention, and FIG. 2 is across-sectional view of a light emitting plane in the embodiment of FIG.1 in an enlarged scale.

As shown in FIGS. 1 and 2, the surface-emitting LED according to thefirst embodiment is made by stacking on a (100)-oriented n-type GaAssubstrate 1 doped with, for example, Si as a donor impurity, insequence, an n-type ZnSe buffer layer 2 doped with, for example, Cl as adonor impurity; an n-type ZnSSe layer 3 doped with, for example, Cl as adonor impurity; an n-type ZnMgSSe cladding layer 4 doped with, forexample, Cl as a donor impurity; an n-type ZnSSe waveguide layer 5 dopedwith, for example, Cl as a donor impurity; an active layer 6; a p-typeZnSSe waveguide layer 7 doped with, for example, N as an acceptorimpurity; a p-type ZnMgSSe cladding layer 8 doped with, for example, Nas an acceptor impurity; a p-type ZnSSe layer 9 doped with, for example,N as an acceptor impurity; a p-type ZnSe contact layer 10 doped with,for example, N as an acceptor impurity; a p-type ZnSe/ZnTemulti-quantum-well layer (MQW) 11 comprising alternately stacked p-typeZnSe barrier layers and p-type ZnTe quantum well layer; and a p-typeZnTe contact layer 12 doped with, for example, N as an acceptorimpurity. A grid-shaped p-side electrode 13 is formed on the p-type ZnTecontact layer 12, and a Au film 14 is provided on the entire surface,covering the p-side electrode 13. Formed on the back surface of then-type GaAs substrate 1 is an n-side electrode 15. The p-type ZnSe/ZnTeMQW layer 11 will be explained later in greater detail.

In the first embodiment, the active layer 6 has a single-quantum-wellstructure or a multi-quantum-well structure including one or morequantum well layers made of ZnCdSe (for example, Zn₀.85 Cd₀.15 Se) of atotal thickness of 6 to 12 nm.

Used as the n-type ZnSSe layer 3 and the p-type ZnSSe layer 9 are, forexample, ZnS₀.06 Se₀.94 layers. Similarly, the n-type ZnSSe waveguidelayer 5 and the p-type ZnSSe waveguide layer 7 are, for example, ZnS₀.06Se₀.94. The n-type ZnMgSSe cladding layer 4 and the p-type ZnMgSSecladding layer 8 are, for example, Zn₀.91 Mg₀.09 S₀.18 Se₀.82 layers.The n-type ZnMgSSe cladding layer 4 and the p-type ZnMgSSe claddinglayer 8 with the composition of Zn₀.91 Mg₀.09 S₀.18 Se₀.82 are inlattice matching with GaAs, and the n-type ZnSSe layer 3, p-type ZnSSelayer 9, n-type ZnSSe waveguide layer 5 and p-type ZnSSe waveguide layer7 having the composition of ZnS₀.06 Se₀.94 are in lattice matching withthe n-type ZnMgSSe cladding layer 4 and the p-type ZnMgSSe claddinglayer 8 having the above composition.

The n-type ZnSSe layer 3 is 0.7 μm thick, for example, and its effectivedonor concentration N_(D) -N_(A) (N_(D) is the donor concentration andN_(A) is the acceptor concentration) is (2˜5)×10¹⁷ cm⁻³, for example.The n-type ZnMgSSe cladding layer 4 is 0.7 μm thick, for example, andits N_(D) -N_(A) is (2˜5)×10¹⁷ cm⁻³, for example. The n-type ZnSSewaveguide layer 5 is 100 μnm thick, for example, and its N_(D) -N_(A) is(2˜5)×10¹⁷ cm⁻³, for example. The p-type ZnSSe waveguide layer 7 is 100nm thick, for example, and its effective acceptor concentration N_(A)-N_(D) is (2˜5)×10¹⁷ cm⁻³, for example. The p-type ZnMgSSe claddinglayer 8 is 0.5 μm thick, for example, and its N_(A) -N_(D) is 1×10¹⁷cm⁻³, for example. The p-type ZnSSe layer 9 is 0.5 μm thick, forexample, and its N_(A) -N_(D) is (2˜5)×10¹⁷ cm⁻³, for example. Thep-type ZnSe contact layer 10 is 100 nm thick, for example, and its N_(A)-N_(D) is (5˜8)×10¹⁷ cm⁻³, for example. The p-type ZnTe contact layer 12is 2˜100 nm thick, for example, and its N_(A) -N_(D) is 1×10¹⁹ cm⁻³, forexample.

The thickness of the n-type ZnSe buffer layer 2 is determinedsufficiently smaller than the critical thickness of ZnSe (˜100 nm) inorder to prevent generation of dislocation during epitaxial growth ofthe n-type ZnSe buffer layer 2 and other overlying layers, which mayoccur due to a lattice mismatching, although slight, between ZnSe andGaAs. In the first embodiment, the thickness of the n-type ZnSe bufferlayer 2 is 33 nm, for example.

Used as the p-side electrode 13 is a Au electrode, Pd/Pt/Au electrode,or the like. The n-side electrode 15 is an In electrode, for example.

The surface-emitting LED according to the first embodiment has a squaregeometry of 1 mm×1 mm, for example, in its plan view.

In the first embodiment, the p-type ZnSSe layer 9 has various functions,such as the behavior as an additional second p-type cladding layer otherthan the p-type ZnMgSSe cladding layer 8; diffusing a current introducedfrom the p-side electrode 13 and the Au film 14 in the directionparallel to plane of the p-type ZnSSe layer 9; establishinglattice-matching with the p-type ZnMgSSe cladding layer 8; and behavingas a spacer for preventing short-circuit which might occur when soldercreeps up along end surfaces of diode chips upon mounting such chips ona heat sink.

The p-type ZnSSe layer 9 behaving as a second p-type cladding layer,together with the p-type ZnMgSSe cladding layer 8, can improve opticalconfinement characteristics and carrier confinement characteristics.Since the mobility of holes in ZnSSe is larger than that in ZnMgSSe, theresistance of the p-type cladding layer is lower in the structure whereboth the p-type ZnMgSSe cladding layer 8 and the p-type ZnSSe layer 9form the p-type cladding layer of a given total thickness than in thestructure where only the p-type ZnMgSSe 8 forms the p-type claddinglayer of the same total thickness. Lower resistance of the p-typecladding layer decreases the voltage drop of the p-type cladding layer,and contributes to a reduction of the operating voltage of thesurface-emitting LED.

The function of diffusing a current introduced from the p-side electrode13 and the Au film 14 in the direction parallel with the plane of thep-type ZnSSe layer 9 makes the current be injected to a wider area ofthe active layer 6, and ensures uniform surface emission of light.

If the p-type ZnSe contact layer 10 is stacked directly on the p-typeZnMgSSe cladding layer 8, a lattice mismatching between them would causetheir crystalline deterioration. However, since the p-type ZnSSe layer 9in lattice matching with the p-type ZnMgSSe cladding layer 8 is stackedthereon, and the p-type ZnSe contact layer 10 is stacked on the p-typeZnSSe layer 9, the p-type ZnSSe layer 9 and p-type ZnSe contact layer 10are maintained in a good crystalline condition. This contributes to animprovement in ohmic contact of the p-side electrode 13 with the Au film14.

In addition to these advantages, the use of the p-type ZnSSe layer 9 hasthe following merit. That is, the use of the p-type ZnSSe layer 9 as thesecond p-type cladding layer permits a minimal thickness of the p-typeZnMgSSe cladding layer 8 whose epitaxial growth is not so easy as thatof binary or tertiary II-VI compound semiconductors, and hence makesfabrication of surface-emitting LEDs so much easier.

The n-type ZnSSe layer 3 has some functions, such as symmetrizingrefractive indices at both sides of the active layer 6; behaving as asecond n-type cladding layer additional to the n-type ZnMgSSe claddinglayer 4; lattice-matching with the n-type ZnMgSSe cladding layer 4; andbehaving as a spacer for preventing short-circuit which might occur whensolder creeps up along end surfaces of diode chips upon mounting suchchips on a heat sink.

Since the p-type ZnTe contact layer 12 is as thin as 2˜100 nm andexhibits a significantly high N_(A) -N_(D) as high as 1×10¹⁹ cm⁻³, itworks as a transparent electrode with a low resistance. This results ina good ohmic contact with the p-side electrode 13 and the Au film 14formed on the p-type ZnTe contact layer 12, while ensuring emission oflight with no disturbance even with the p-type ZnTe contact layer 12covering the entire surface. When the thickness of the p-type ZnTecontact layer 12 is around 50 nm, a current introduced through thep-side electrode 13 and the Au film 14 can diffuse widely in thedirection parallel with the plane of the p-type ZnTe contact layer 12,which contributes to realization of excellently uniform surface emissionof light.

If the p-type ZnSe contact layer 10 were in direct contact with thep-type ZnTe contact layer 12, large discontinuity would be produced invalence bands near their interface and would behave as a barrier layeragainst holes injected from the p-side electrode 13 and the Au film 14into the p-type ZnTe contact layer 12. The use of the p-type ZnSe/ZnTeMQW layer 11 is for the purpose of effectively removing the barrier.

More specifically, although the carrier concentration in p-type ZnSe istypically on the order of 5×10¹⁷ cm⁻³, the carrier concentration inp-type ZnTe can be 10¹⁹ cm⁻³ or more. In addition, the magnitude of thediscontinuity in valence bands at the interface between p-type ZnSe andp-type ZnTe is about 0.5 eV. At the junction between p-type ZnSe andp-type ZnTe, if it is a step junction, the valence band of p-type ZnSebends over the width

    W=(2εφ.sub.T /qN.sub.A).sup.1/2                (1)

where ε is the dielectric constant of ZnSe, and φ_(T) is the dimensionof the discontinuity in valence bands at the p-type ZnSe/p-type ZnTeinterface (about 0.5 eV).

Calculation of W using Equation (1) results in W=32 nm. FIG. 3 shows howthe top of a valence band changes in the direction normal to theinterface between p-type ZnSe and p-type ZnTe interface. Note that Fermilevels of p-type ZnSe and p-type ZnTe are similar when coinciding withtops of their valence bands. In this case, the valence band of p-typeZnSe bends downwardly (toward lower energies) toward p-side ZnTe asshown in FIG. 3. This change in valence band having a downwardly pointedrepresentation behaves as a potential barrier against holes injectedinto the p-side ZnSe/p-side ZnTe junction.

This problem can be solved by providing the p-type ZnSe/ZnTe MQW layer11 between the p-type ZnSe contact layer 10 and the p-type ZnTe contactlayer 12. A specific design of the p-type ZnSe/ZnTe MQW layer 11 isexplained below.

FIG. 4 shows a result of quantum-mechanical calculation of a well-typepotential of a finite barrier to know how the first quantum level E₁changes with the width L_(w) of the quantum well of p-type ZnTe in asingle-quantum-well structure in which the p-type ZnTe quantum welllayer is sandwiched by p-type ZnSe barrier layers from opposite sides.The calculation uses 0.6 m₀ (m₀ : stationary mass of an electron invacuum) as the mass of an electron in the quantum well layer and thebarrier layer, letting it be equal to the effective mass m_(h) of a holein p-type ZnSe and p-type ZnTe, and assumes the depth of the well being0.5 eV.

It is known from FIG. 4 that, by decreasing the width L_(w) of thequantum well, the first quantum level E₁ formed in the quantum well canbe elevated. The p-type ZnSe/ZnTe MQW layer 11 is designed, using thistheory.

In this case, the bend in the band of p-type ZnSe over the width W fromthe interface between p-type ZnSe and p-type ZnTe is given by thefollowing quadratic function of the distance x (FIG. 3) from theinterface

    φ(x)=φ.sub.T {1-(x/W).sup.2 }                      (2)

Therefore, the p-type ZnSe/ZnTe MQW layer 11 can be designed, based onEquation (2), by stepwise changing L_(w) such that first quantum levelsE₁ formed in respective p-type ZnTe quantum well layers coincide withtop energies of valence bands of p-type ZnSe and p-type ZnTe, and areequal to each other.

FIG. 5 shows a design of the widths L_(w) of quantum wells of the p-typeZnSe/ZnTe MQW layer 11 when the width of its each p-type ZnSe barrierlayer is 2 nm. In this case, N_(A) -N_(D) of the p-type ZnSe contactlayer 10 is 5×10¹⁷ cm⁻³, and N_(A) -N_(D) of the p-type ZnTe contactlayer 12 is 1×10¹⁹ cm⁻³. As shown in FIG. 5, widths L_(w) of sevenquantum wells, in total, are made to vary such that their first quantumlevels E₁ coincide with Fermi levels of p-type ZnSe and p-type ZnTe,that is, to vary from the p-type ZnSe contact layer 10 toward the p-typeZnTe contact layer 12 as L_(w) =0.3 nm, 0.4 nm, 0.5 nm, 0.6 nm, 0.8 nm,1.1 nm, and 1.7 nm.

Upon designing widths L_(w) of quantum wells, in a strict sense,interactions among the quantum wells must be considered because theirlevels couple with each other, and effects of any possible distortiondue to lattice mismatching between quantum wells and barrier layers mustbe incorporated as well. Theoretically, however, it is sufficientlypossible to make the quantum levels of the multi-quantum-wells flat asshown in FIG. 5.

In FIG. 5, since holes injected into p-type ZnTe can flow to p-type ZnSeby resonant tunneling through first quantum levels E₁ formed inrespective quantum wells in the p-type ZnSe/ZnTe MQW layer 11,substantially no potential barrier is produced at the interface betweenp-type ZnSe and p-type ZnTe. Therefore, the surface-emitting LEDaccording to the first embodiment of the invention exhibits goodvoltage-to-current characteristics and is operative with a lowervoltage.

To operate the surface-emitting LED according to the first embodimenthaving the above construction, a current is injected by applying anecessary voltage between the p-side electrode 13 as well as the Au film14 and the n-side electrode 15. In this case, due to the p-sideelectrode 13 and the Au film 14 being in contact with the entire surfaceof the p-type ZnTe contact layer 12, the current is uniformly injectedto the entire area of the p-type ZnTe contact layer 12 from the p-sideelectrode 13 and the Au film 14. Moreover, while passing the p-type ZnTecontact layer 12 and the p-type ZnSSe layer 9, the current issufficiently distributed in directions parallel to the planes of theselayers. Therefore, the current is injected to the entirety of the activelayer 6 with a uniform distribution, which results in uniform emissionof light by electron-hole recombination-in the entirety of the activelayer 6. Thus, as shown by the arrow in FIG. 2, light with an extensivediameter is taken out from the light emitting surface 16 proximal to thep-side electrode 13, and remarkably uniform surface emission of light isrealized.

The surface-emitting LED according to the first embodiment was operatedfor a test with the injected current of 200 mA at room temperature, andsurface emission of bluish green light with the wavelength of 512 nm wasobserved. The luminous intensity of the light was remarkably high, ashigh as 4 cd. The light output was 1.14 mW, and the external quantumefficiency was 2.35%. The width at half maximum of the peak of emissionwith the wavelength of 512 nm was 10 nm.

Explained below is a method for fabricating the surface-emitting LEDaccording to the first embodiment having the above-explainedconstruction.

To fabricate the surface-emitting LED according to the first embodiment,first epitaxially grown on the n-type GaAs substrate 1 by molecular beamepitaxy (MBE), for example, at a temperature in the range of e.g. 250 to300° C., specifically at 295° C., for example, are in sequence: then-type ZnSe buffer layer 2, n-type ZnSSe layer 3, n-type ZnMgSSecladding layer 4, n-type ZnSSe waveguide layer 5, active layer 6including ZnCdSe quantum well layers, p-type ZnSSe waveguide layer 7,p-type ZnMgSSe cladding layer 8, p-type ZnSSe layer 9, p-type ZnSecontact layer 10, p-type ZnMgSSe cladding layer 8, p-type ZnSe/ZnTe MQWlayer 11, and p-type ZnTe contact layer 12. This process ensuresepitaxial growth of these layers with good crystalline structures, hencediminishes deterioration of the surface-emitting LED such as a decreasein light output, and ensures a high reliability of the device.

Used for the epitaxial growth using MBE are Zn with the purity of99.9999% as the material of Zn, Mg with the purity of 99.9% or more asthe material of Mg, ZnS with the purity of 99.9999% as the material ofS, and Se with the purity of 99.9999% as the material of Se. Doping ofCl as a donor impurity of the n-type ZnSe buffer layer 2, n-type ZnSSelayer 3, n-type ZnMgSSe cladding layer 4 and n-type ZnSSe waveguidelayer 5 is done, using ZnCl₂ with the purity of 99.9999%, for example,as the dopant. On the other hand, doping of N as an acceptor impurity ofthe p-type ZnSSe waveguide layer 7, p-type ZnMgSSe cladding layer 8,p-type ZnSSe layer 9, p-type ZnSe contact layer 10, p-type ZnSe/ZnTe MQWlayer 11 and p-type ZnTe contact layer 12 is done by irradiating N₂plasma generated by electron-cyclotron resonance (ECR), for example.

After that, a resist pattern (not shown) with a geometry correspondingto the reverse pattern of the p-side electrode 13 is formed on thep-type ZnTe contact layer 12 by lithography, followed by deposition of ametal film for making the p-side electrode on the entire surface of thestructure by, for example, sputtering or vacuum evaporation. Then theresist pattern is removed together with overlying portions of the metalfilm (lift-off). Thus the grid-shaped p-side electrode 13 is formed onthe p-type ZnTe contact layer 12. After that, annealing is done, ifnecessary, to bring the p-side electrode 13 and the Au film 14 intoohmic contact with the p-side ZnTe contact layer 12. Formed on the backsurface of the n-type GaAs substrate 1 is the n-side electrode 15, suchas In electrode.

After that, the n-type GaAs substrate 1 supporting the diode structurethereon is cleaved into cubes of the size of 1 mm×1 mm to completeexpected surface-emitting LEDs.

Epitaxial growth of respective layers of the surface-emitting LEDaccording to the first embodiment may use metallorganic chemical vapordeposition in lieu of MBE.

As described above, the first embodiment can realize a surface-emittingLED capable of surface emission of bluish green light with a highluminance and a high performance by using II-VI compound semiconductors.

FIG. 6 shows a surface-emitting LED according to a second embodiment ofthe invention. Plan view of this surface-emitting LED appears identicalto FIG. 1.

As shown in FIG. 6, the surface-emitting LED according to the secondembodiment has the same construction as that of the surface-emitting LEDaccording to the first embodiment, except for its p-type ZnSSe layer 9being as thick as several times μm; selective portions of the p-typeZnSe/ZnTe MQW layer 11 and the p-side ZnTe contact layer 12 other thanthose lying under the p-side electrode 13 being partly removed to exposethe underlying p-type ZnSe contact layer 10 on which an anti reflectionfilm 17 of e.g. SiN is formed; and no Au film 14 being used.

The surface-emitting LED according to the second embodiment can befabricated by substantially the same method as that according to thefirst embodiment, which is omitted from the explanation made below.

Since the surface-emitting LED according to the second embodiment usesno Au film 14, electric current is introduced only through the p-sideelectrode 13, which results in less uniform distribution of the currentnear the p-side electrode 13 than that of the surface-emitting LEDaccording to the first embodiment. However, since the current candiffuse wider and wider in the directions parallel to the plane of thep-side ZnSSe layer 9 while passing through the layer 9 which is as thickas several times μm, the current can extend even to the central portionof the active layer 6 which is offset from the p-side electrode 13. InFIG. 6, this aspect is schematically shown by paths of holes. In thisfashion, excellently uniform surface emission of light can be realizedeven though the current is injected only through the grid-shaped p-sideelectrode 13.

Also the second embodiment, as well as the first embodiment, can realizea surface-emitting LED capable of surface emission of bluish green lightwith a remarkably high luminance and a high performance by using II-VIcompound semiconductors.

FIG. 7 shows a surface-emitting LED according to a third embodiment ofthe invention. Plan view of this surface-emitting LED appears identicalto FIG. 1.

As shown in FIG. 7, the surface-emitting LED according to the thirdembodiment includes a Bragg reflector 18 comprising a ZnMgSSe/ZnSSesuperlattice layer provided between the n-type ZnSSe layer 3 and then-type ZnMgSSe cladding layer 4. To maximize the reflectivity of theBragg reflector 18, the thickness of each layer of the ZnMgSSe/ZnSSesuperlattice layer forming the Bragg reflector 18 is determined suchthat the optical distance obtained by multiplying the thickness by therefractive index be 1/4 of the wavelength of emission. To furtherincrease the reflectivity of the Bragg reflector 18, it is recommendedto increase the period of repetition of the ZnMgSSe/ZnSSe superlatticelayer forming the Bragg reflector 18. In the other respects, thesurface-emitting LED according to the third embodiment is the same asthe arrangement of the surface-emitting LED according to the firstembodiment.

The surface-emitting LED according to the third embodiment can befabricated by substantially the same method as that according to thefirst embodiment, which is omitted from the explanation made below.

According to the surface-emitting LED according to the third embodiment,since a component of light generated in the active layer 6 and runningtoward the n-type GaAs substrate 1 is reflected by the Bragg reflector18 toward the p-side electrode 13, it is prevented that the light towardthe n-type GaAs substrate 1 is absorbed by the n-type GaAs substrate 1,and even the light toward the n-type GaAs substrate can be utilized forsurface emission. As a result, emission efficiency can be approximatelydoubled as compared with a structure using no Bragg reflector 18.

The third embodiment also realizes a surface-emitting LED capable ofsurface emission of bluish green light with a higher luminance thanthose of the first and second embodiments.

In a device using the Bragg reflector 18, a disturbance may occur for areduction of the operating voltage due to a voltage drop caused by theBragg reflector 18. However, by taking an appropriate transaction, suchas inclining the composition ratio at the heterointerface of theZnMgSSe/ZnSSe superlattice layer forming the Bragg reflector 18, dopinga high concentration of impurity into the MgSSe/ZnSSe superlatticelayer, or executing so-called delta doping to form a microcapacitor,voltage drop caused by the Bragg reflector 18 during actual operation ofthe surface-emitting LED can be decreased. Then the surface-emitting LEDcan be protected from deterioration and be operative for a longer life.

Having described specific preferred embodiments of the present inventionwith reference to the accompanying drawings, it is to be understood thatthe invention is not limited to those precise embodiments, and thatvarious changes and modifications may be effected therein by one skilledin the art without departing from the scope or the spirit of theinvention as defined in the appended claims.

For example, a surface-emitting semiconductor laser can be realized withthe same structure as that of the surface-emitting LED according to thethird embodiment. That is, the Bragg reflector 18 and vacuum on the sideof the p-side electrode 13 make a vertical cavity structure capable oflaser oscillation.

The n-type ZnSSe waveguide layer 5 and the p-type ZnSSe waveguide layer7 used in the first to third embodiments may be replaced by an n-typeZnSe waveguide layer and a p-type ZnSe waveguide layer. Alternativelyusable as the substrate is, for example, a GaAs substrate, ZnSesubstrate, GaP substrate, or the like.

Although the first to third embodiments employ irradiation of N₂ plasmagenerated by ECR for doping N as an acceptor impurity into the p-typeZnSe waveguide layer 7, p-type ZnMgSSe cladding layer 8, p-type ZnSSelayer 9, p-type ZnSe contact layer 10, p-type ZnSe/ZnTe MQW layer 11 andp-type ZnTe contact layer 16, doping of N may be done by irradiating N₂excited by high frequency plasma, for example.

Since the surface-emitting semiconductor light emitting device accordingto the invention can emit green to blue light with a high luminance, bycombining this device with an existing semiconductor light emittingdevices for emitting red light with a high luminance, three primarycolors can be made. Thus a color display or other like system can berealized.

As described above, the invention can realize a surface-emittingsemiconductor light emitting device capable of surface emission of greento blue light, using II-VI compound semiconductors as materials of itscladding layers and active layer.

What is claimed is:
 1. A surface-emitting semiconductor light emittingdevice, comprising:a first cladding layer on a substrate; an activelayer on said first cladding layer; a second cladding layer on saidactive layer; a first electrode electrically connected to said firstcladding layer; a second electrode electrically connected to said secondcladding layer; and a ZnTe layer between said active layer and saidsecond electrode, said ZnTe layer being provided in a region other thana light emitting region; said first cladding layer, said active layerand said second cladding layer comprising II-VI compound semiconductors,and light being emitted in a direction normal to the plane of saidactive layer from one side of the active layer remoter from saidsubstrate.